Presently, raster scan printers are called upon to produce binary pixel half-tone images from full grey scale pixel images. In a grey scale image, each pixel element fed from a processor includes a multi-bit binary value that defines the pixel's grey scale value. An eight bit byte can define any of 0-255 grey scale values. In converting such a grey scale pixel image to a binary value pixel image, each pixel's grey scale value is compared to a threshold, and if the grey scale value equals or exceeds the threshold, it is assigned a first binary value (e.g. a "one"). If its value is less than the threshold, it is assigned a complementary binary value (e.g. a "zero"). Such conversion accomplishes the creation of a binary half tone image, but the quality of the image is less than satisfactory as it does not account for error values that occur during the conversion. An "error value" is the difference between the binary pixel value after conversion and the grey scale value of the pixel before conversion.
To improve the quality of binary images, Floyd et al. in "An Adaptive Algorithm Spatial Grey Scale" Proceedings of the Society for Information Display: Vol. 17, No. 75 (1976) pp. 36-37 suggested that error values be diffused to adjacent pixels so as to provide smoother half-tone transitions and a more pleasing picture. In FIGS. 1 and 2, the error diffusion algorithm described by Floyd et al. will be considered.
In FIG. 1, each pixel in grey scale image 10 has an assigned grey scale value. Exemplary values are indicated above each column of pixels and it is assumed that all pixels in each column have the same grey scale value. Accordingly, pixel 12 (P.sub.i) has a grey scale value of 20 (assuming a grey scale range of 0-255). If it is further assumed that a threshold for conversion of grey scale values to binary values is midway between 0 and 255, i.e., 127, then pixel 12, upon comparison to the threshold, is converted to a zero binary value. However, this conversion creates a grey scale error value of 20 (when the binary pixel value is compared to the grey scale value of pixel 12 prior to conversion). Floyd et al. suggest an allocation of the pixel's error to immediately adjacent pixels.
An enlarged view of pixel P.sub.i and its adjacent pixels on scan lines n-1, n, and n+1 is shown in FIG. 2. The Floyd et al. error diffusion procedure considers only the pixel to the right of pixel 12 and immediately adjacent pixels 14, 16 and 18 on the next scan line. Floyd et al. consider that all pixels on scan line n-1 and those pixels to the left of P.sub.i have already been processed and can no longer be modified.
Floyd et al. allocate defined fractions of the pixel error from pixel 12 to adjacent pixels 14, 16, 18 and 20 as follows: 1/16, 5/16, 3/16 and 7/16, respectively. The allocation of the pixel error to neighboring pixels causes a cancellation of the error values and creates a total brightness of the pixel group which, when taken together, is close to the total desired brightness.
Prior art processors which implement the Floyd et al. error diffusion algorithm do so in a time consuming manner. Classically, each pixel to which an error value is to be attributed is accessed from memory; the error value attributable to that pixel calculated; the attributed error value added to the pixel value; and the sum returned to memory. When such a pixel is later reached for conversion to a binary value, identical computations are performed for each pixel that adjoins it, etc. Thus, for each pixel, an error diffusion calculation involves at least four memory accesses and four independent error value calculations. In a high speed printer, the available time for error value diffusion is constrained and, insufficient processor time is available to perform the prior art procedures for implementing the Floyd et al. error diffusion procedure.
Accordingly, it is an object of this invention to provide an improved implementation of the Floyd et al. error diffusion procedure.
It is another object of this invention to provide an implementation of the Floyd et al. error diffusion procedure which reduces required memory accesses.